It is not easy to install a furnace with do-it-yourself instructions due to the skill, precision and tools required. An unlicensed individual should not attempt installation no matter the potential savings. Only licensed professionals should install a furnace, as incorrect installation poses a safety hazard to the home and family.
Furnaces operate on natural gas, electric, propane or oil, and the type of fuel the furnace consumes dictates the method of installation. If there is pre-existing ductwork that is efficient and without air leaks, then the labor is less intensive. The furnace needs proper ventilation and exhaust, as well as clearance for convenient repair and maintenance.